There are three types of electrolytic cells commercially used for producing halogen gas and aqueous caustic solutions from alkali metal halide brines, a process referred to by industry as a chlor-alkali process. The three types of cells are: (1) a mercury cell, (2) a diaphragm cell and (3) a membrane cell. The general operation of each cell is known to those skilled in the art and is discussed in Volume 1 of the Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Ed. (John Wiley & Sons 1978) at page 799 et. seq., the relevant teachings of which are incorporated herein by reference.
The three cells differ in various respects. In the mercury cell, alkali metal ions produced by electrolysis of an alkali metal salt form an amalgam with mercury. The amalgam reacts with water to produce aqueous sodium hydroxide, hydrogen gas and free mercury. The mercury is recovered and recycled for further use as a liquid cathode. In a diaphragm cell, an alkali metal halide brine solution is fed into an anolyte compartment where halide ions are oxidized to produce halogen gas. Alkali metal ions migrate into a catholyte compartment through a hydraulically-permeable microporous diaphragm disposed between the anolyte compartment and the catholyte compartment. Hydrogen gas aqueous alkali metal hydroxide solutions are produced at the cathode. Due to the hydraulically-permeable diaphragm, brine may flow into the catholyte compartment and mix with the alkali metal hydroxide solution. A membrane cell functions similar to a diaphragm cell, except that the diaphragm is replaced by a hydraulically-impermeable, cationically-permselective membrane which selectively permits passage of hydrated alkali metal ions to the catholyte compartment. A membrane cell produces aqueous alkali metal hydroxide solutions essentially uncontaminated with brine. Presently, the most widely used chlor-alkali processes employ either diaphragm of membrane cells.
The minimum voltage required to electrolyze a sodium chloride brine into chlorine gas, hydrogen gas and aqueous sodium hydroxide solution may be theoretically calculated by the use of thermodynamic data. However, in reality, production at the theoretical voltage is not attainable and a higher voltage, i.e., a so-called overvoltage, must be applied to overcome various inherent resistances within the cell. Reduction in the amount of applied overvoltage leads to a significant savings of energy costs associated with cell operation. A reduction of even as little as 0.05 volts in the applied overvoltage translates to significant energy savings when processing multimillion-ton quantities of brine. As a result, it is desirable to discover methods which minimize overvoltage requirements.
Throughout the development of chlor-alkali technology, various methods have been proposed to reduce the overvoltage requirements. To decrease the overvoltage in a diaphragm or a membrane cell, one may attempt to reduce electrode overvoltages, i.e., a so-called hydrogen overvoltage at the cathode; to reduce electrical resistance of the diaphragm or membrane; to reduce electrical resistance of the brine being electrolyzed; or to use a combination of these approaches. Some research concentrates on minimizing cell overvoltage by proposing design modifications to the cells.
It is known that the overvoltage for an electrode is a function of its chemical characteristics and current density. See, W. J. Moore, Physical Chemistry, pp. 406-408 3rd Ed. (Prentice Hall 1962). Current density is defined as the current applied per unit of actual surface area on an electrode. Techniques which increase the actual surface area of an electrode, such as acid etching or sandblasting the surfaces thereof, result in a corresponding decrease of the current density for a given amount of applied current. Inasmuch as the overvoltage and current density are directly related to each other, a decrease in current density yields a corresponding decrease in overvoltage. The chemical characteristics of materials used to fabricate the electrode also impact overvoltage. For example, electrodes incorporating an electrocatalyst accelerate kinetics for electrochemical reactions occurring at the surface of the electrode.
It is known that certain platinum group metals, such as ruthenium, rhodium, osmium, iridium, palladium, platinum, and oxides thereof are useful as electrocatalysts. Electrodes may be fabricated from these metals, but more economical methods affix the platinum group metals to a conductive substrate such as steel, nickel, titanium, copper and so on. For example, U.S. Pat. No. 4,414,071 discloses coatings of one or more platinum group metals deposited as a metallic layer on an electrically-conductive substrate. Japanese Patent No. 9130/65, OPI application numbers 131474/76 and 11178/77, refers to use of a mixture of at least one platinum group metal oxide with a second metal oxide as a cathode coating.
Also known in the art are coatings of catalytic metals in both an elemental and combined form. U.S. Pat. Nos. 4,724,052 and 4,465,580 are similar and teach preparation of a coating on a metallic substrate by electrolytic deposition of catalytic metals and catalytic particles thereon. U.S. Pat. No. 4,238,311 teaches a cathode coating consisting of fine particles of platinum group metals, platinum group metal oxides or a combination thereof, affixed to a nickel substrate. Such processes are undesirable due to either the need for expensive electrolytic hardware or waste disposal problems.
Some research has concentrated on cathodes having layered catalyst coatings. U.S. Pat. No. 4,668,370 discusses a coating having an interlayer deposited by electrolytic deposition, the interlayer being an inert metal with particles of a ceramic material, such as platinum group metal oxides, dispersed therein. On top of the interlayer is a layer of ceramic material which includes metal oxides. U.S. Pat. No. 4,798,662 discloses a coating having a base layer that includes the platinum group metals, metal oxides and mixtures thereof. On top of this base layer is a layer of metal, such as nickel or cobalt.
Industry has recently directed attention toward development of "zero-gap" electrolytic cells wherein an electrode, such as the cathode, is placed in contact with a membrane. This arrangement reduces the required overvoltage of prior "gap" cell designs by elimination of electrical resistance caused by electrolyte being disposed between the cathode and the membrane. In some zero-gap cells, it is advantageous to employ an extremely thin cathode to provide close contact between the cathode and the membrane and, thereby, fully utilize the advantage of the zero-gap cell design. A thin substrate also provides flexibility, which helps prevent damage to the membrane caused by contact with the cathode. However, use of a thin substrate presents problems in maintaining adherence of electrocatalytic coatings to the substrate. Substrates coated by prior methods can experience significant coating loss by decrepitation shortly after being placed in service, especially where the substrate is flexible. Thin substrates coated by electrolytic methods as previously described also tend to become rigid and lose flexibility. Accordingly, it is desirable to develop a coating which is both resistant to loss during operation and which allows for retention of substrate flexibility.
Coatings of catalytic metals possessing low hydrogen overvoltage properties are typically subject to loss of catalytic activity due to poisoning by inherent impurities present in electrolyte solutions. For example, contaminants present in commercial-scale electrolytic cells, such as iron in an ionic form, may be reduced at the cathode and will eventually plate over a catalytic metal coating. Over a period of time, catalyst performance degrades and results in the cathode performing at an overvoltage level equivalent to a cathode fabricated from the metal impurity. The so-called hydrogen overvoltage, an indicator of cathode performance used by those skilled in the art of electrolysis, for iron is quite high at current densities of 1.5 to 3.5 amps per square inch typically employed in commercial chlor-alkali cells. In contrast, it is desirable to maintain a low hydrogen overvoltage, as generally exhibited by the favorable low hydrogen overvoltage for platinum group metals and platinum group metal oxides, during long-term operation of the cell.
Generally, the cathodes disclosed in the above-identified patents are prepared prior to their use and assembly within an electrolytic cell. These cathodes can require expensive equipment and extensive amounts of labor for their preparation. Further, if a cathode loses catalytic activity due to poisoning, a considerable amount of cell down-time and costs may be required to replace it. Poisoning may even result in the need to discard the cathode.
It is, therefore, desirable to develop a cathode possessing a low hydrogen overvoltage that is resistant to poisoning by impurities. It is also desirable that the catalyst be tightly adhered to the substrate to inhibit its loss during operation and, thereby, maintain a low hydrogen overvoltage for the cell. It would also be desirable to develop a method for reducing cell hydrogen overvoltage by preparing or regenerating an activated cathode in situ, i.e., while the cathode is assembled within a cell.